Recently, the application of very small elements produced through a micromachining technique has been expanding in various technical fields. An example of such elements is an angular speed sensor that includes a minute oscillating portion. The angular speed sensor is employed for shake compensation in a digital camera or a video camera against the user's hand motion, for a car navigation system, or for controlling the posture of a vehicle or a robot. Description of such angular speed sensor can be found, for example, in the following patent documents 1-3.
1. Japanese Laid-open Patent Publication No. 2003-28648
2. Japanese Laid-open Patent Publication No. 2004-163374
3. Japanese Laid-open Patent Publication No. 2007-33330
FIG. 26 depicts an angular speed sensor X5, as an example of angular speed sensor as related art. The angular speed sensor X5 includes a supporting substrate S2, oscillating portions 80A, 80B, a plurality of fixing posts 81, driving electrodes 82A, 82B, 83A, 83B, 84A, 84B, 85A, 85B, detecting electrodes 86A, 86B, 87A, 87B, a coupling beam 88, and link portions 89A, 89B. In FIG. 26, the link portions 89A, 89B located with a spacing from the supporting substrate S2 are hatched, and other portions spaced from the supporting substrate S2 other than the link portions 89A, 89B are solidly filled, for the sake of explicitness of the drawing. The fixing posts 81, the electrode pads of the driving electrodes 82A, 82B, 84A, 84B, and the electrode pads of the detecting electrodes 86A, 86B, 87A, 87B are fixed on the supporting substrate S2.
The oscillating portions 80A, 80B each include a comb electrode structure, and are capable of oscillating in an X-axis direction, as well as in a Y-axis direction. The X-axis direction and the Y-axis direction are perpendicular to each other. The fixing post 81 is provided upright on the supporting substrate S2. The driving electrodes 82A, 83A, 84A, 85A each include a comb electrode structure, and constitute a driving unit that generates a driving force to cause the oscillating portion 80A to produce reference oscillation in the X-axis direction. The driving electrodes 82B, 83B, 84B, 85B each include a comb electrode structure, and constitute a driving unit that generates a driving force to cause the oscillating portion 80B to produce the reference oscillation in the X-axis direction. The detecting electrodes 86A, 87A each include a comb electrode structure, and constitute a detector that detects displacement of the oscillating portion 80A in the Y-axis direction. The detecting electrodes 86B, 87B each include a comb electrode structure, and constitute a detector that detects displacement of the oscillating portion 80B in the Y-axis direction. The coupling beam 88 serves to couple the respective reference oscillation of the oscillating portions 80A, 80B such that the reference oscillation is caused in the opposite phases. The link portion 89A serves as a bridge between the oscillating portion 80A, a predetermined fixing post 81, the driving electrodes 83A, 85A, and the coupling beam 88, to thereby transmit the driving force generated at the driving electrodes 82A, 83A, 84A, 85A to the oscillating portion 80A and the coupling beam 88, while supporting the oscillating portion 80A. The link portion 89B serves as a bridge between the oscillating portion 80B, a predetermined fixing post 81, the driving electrodes 83B, 85B, and the coupling beam 88, to thereby transmit the driving force generated in the driving electrodes 82B, 83B, 84B, 85B to the oscillating portion 80B and the coupling beam 88, while supporting the oscillating portion 80B. The oscillating portions 80A, 80B, the driving electrodes 83A, 83B, 85A, 85B, and the coupling beam 88 constitute a movable portion in the angular speed sensor X5.
When the angular speed sensor X5 is driven, a voltage is applied to the driving electrodes 82A, 82B, 83A, 83B, 84A, 84B, 85A, 85B to thereby generate the driving force so that, ideally, the oscillating portions 80A, 80B cause the reference oscillation in the X-axis direction in opposite phases, as illustrated in FIG. 27. To be more detailed, a driving voltage is applied to the driving electrode 82A, 84B in a first period with a predetermined voltage being applied to the movable portion (including the driving electrodes 83A, 83B, 85A, 85B), to thereby generate the driving force between the driving electrodes 82A and 83A and between the driving electrodes 84B and 85B in the first period, and the driving voltage is applied to the driving electrodes 84A, 82B in a second period (ideally, at the same frequency as in the first period and half a period shifted from the first period in phase), to thereby generate the driving force between the driving electrodes 84A and 85A and between the driving electrodes 82B and 83B in the second period.
Under the state where the oscillating portions 80A, 80B are producing the reference oscillation in the X-axis direction, once an angular speed about a Z-axis is exerted to the angular speed sensor X5, and hence to the oscillating portions 80A, 80B, a Coriolis force that displaces the oscillating portions 80A, 80B in the Y-axis direction is periodically generated. Accordingly, the oscillating portions 80A, 80B each oscillate in the Y-axis direction (Coriolis oscillation), and a static capacitance between the oscillating portion 80A and the detecting electrodes 86A, 87A and that between the oscillating portion 80B and the detecting electrodes 86B, 87B are caused to fluctuate. Based on such fluctuation of the static capacitance, the displacement amount, in other words the amplitude of oscillation of the oscillating portions 80A, 80B is detected, so that the angular speed exerted on the angular speed sensor X5, and hence to the oscillating portions 80A, 80B is led out according to the detection result.
In the angular speed sensor X5, the pair of oscillating portions 80A, 80B may suffer noise oscillation for some reasons. However, by regulating the oscillating portions 80A, 80B to produce the reference oscillation with a predetermined phase difference (ideally in opposite phases), the oscillating portions 80A, 80B produce the Coriolis oscillation in the Y-axis direction with the predetermined phase difference (ideally in opposite phases) when the angular speed is exerted. Thus, it is possible to cancel the noise oscillation in detecting the displacement amount of the oscillating portions 80A, 80B. It is for such reason that the angular speed sensor X5 is provided with the oscillating portions 80A, 80B that can be made to perform the reference oscillation in opposite phases.
The coupling beam 88 in the angular speed sensor X5 serves to optimize as much as possible the phase difference in the reference oscillation of the oscillating portions 80A, 80B while they are driven (in other words, adjust the phase to be as close as possible to opposite, which is the ideal state). Without the coupling beam 88, it would be quite difficult to regulate the oscillating portions 80A, 80B so as to accurately produce the reference oscillation in opposite phases while being driven. This is because the natural oscillation frequency of the respective oscillating portions 80A, 80B is actually different, primarily because of a manufacturing tolerance of each constituent of the movable portion including the oscillating portions 80A, 80B. The structure that the coupling beam 88 mechanically connects the oscillating portions 80A, 80B via the link portions 89A, 89B allows the respective reference oscillation of the oscillating portions 80A, 80B to be mutually mechanically associated, so that the phase difference of the reference oscillation of the oscillating portions 80A, 80B can be optimized while they are driven.
In the angular speed sensor X5, however, in the case where, on the assumption that the coupling beam 88 is not provided, the phase difference in the reference oscillation is largely shifted from the ideal state when the oscillating portions 80A, 80B are individually caused to produce the reference oscillation, it may be no longer possible to optimize the phase difference of the reference oscillation to be sufficiently close to opposite, which is ideal, despite employing the coupling beam 88. In other words, the structure that the coupling beam 88 is disposed between the oscillating portions 80A, 80B may still fail to couple the respective reference oscillation of the oscillating portions 80A, 80B with sufficiently high efficiency. The decline in coupling efficiency leads to weakened mechanical association of the respective reference oscillation of the oscillating portions 80A, 80B, and to an increase in driving force necessary for generating the reference oscillation in a predetermined amplitude, which is undesirable from the viewpoint of reducing the driving voltage.
In the angular speed sensor X5, further, the driving energy that drives the oscillating portions 80A, 80B to produce the reference oscillation is prone to leak into the supporting substrate S2 through the fixing post 81. This is because the driving force generated at the driving electrodes 82A, 82B, 83A, 83B, 84A, 84B, 85A, 85B can be transmitted through the link portions 89A, 89B not only to the oscillating portions 80A, 80B but also to the fixing post 81. The leakage of a greater portion of the driving energy into the supporting substrate S2 leads to requirement of an increased driving force for generating the reference oscillation in a predetermined amplitude, which is undesirable from the viewpoint of reducing the driving voltage.